The present invention generally relates to an image forming arrangement and more particularly, to a copying apparatus capable of effecting a simultaneous multi-color copying control for obtaining images in a plurality of colors by causing a plurality of developing units to function through change-over during on copying operation.
Conventionally, there has been proposed a copying apparatus of the above described type, for example, in U.S. patent application No. 148,423 assigned to the same assignee as in the present invention.
In the above known copying apparatus, two magnetic brush type developing units are provided at side portions of a photosensitive or photoreceptor drum which functions as an electrostatic latent image holding member, with developing materials or developers in different colors being accommodated in the respective developing units.
Each of the above developing units is provided with a developing sleeve confronting said photoreceptor drum, and a magnet roller accommodated within said developing sleeve is arranged to be rotatable by a predetermined angle, so that magnetic poles provided on an outer periphery of said magnet roller may be moved or changed over between a developing position confronting said photoreceptor drum and another developing position where an intermediate portion between neighboring magnetic poles faces the photoreceptor drum.
In the conventional copying apparatus having the construction as described above, a portion from a leading edge of the original document to a first boundary region as designated is developed, for example in black through employment of the first developing unit, and successively, another portion from said first boundary region to a second boundary region is developed, for example in color by the use of the second developing unit, and a multi-color image can be obtained by repeating the similar operations as above thereafter.
However, in the above copying apparatus, for the development at the boundary region, turning off of the developing sleeve and change-over of the magnet roller are effected simultaneously, and therefore, inertia force due to speed retardation and stopping of the developing sleeve, and transporting force of the developer following movement of the magnet roller are applied to the developer at the same time, whereby the developer on the developing sleeve is temporarily increased in density, with the toner scattering over the electrostatic latent image holding member, thus resulting in such a problem as reduction of image quality due to noises appearing on the image.